Koi pond aeration options

Choosing a method of aeration for your koi pond is quite different from say choosing a pond pump or UVc. Firstly, not all ponds may need additional aeration. How you currently manage the water movement through your system will have a bearing on whether you choose to add further aeration. Existing vigorous water movement over water falls or through fountains will actively aerate your pond water anyway. Secondly, again in contrast to choosing a pond pump, if you do opt for an aeration device, there are completely different methods to choose from.

Choose you pond perfect aerator from these ones …

What different types of aeration device are available?

a. Diffused aeration. This uses an air pump that delivers air under pressure to a diffuser of some kind.

b. Venturi. A cylindrical hollow tube through which water is pumped. The unique shape through which the water flows draws in atmospheric air, aerating the return flow.

c. Moving water (Fountains / waterfalls). Oxygen dissolves readily into the dynamic and energised water.

What is the job of an aeration device in a Koi pond set up?

Increased oxygenation (surprise surprise!). If we have been successful in developing a mature and stable pond environment, those organisms that essentially maintain the water quality have an insatiable demand for oxygen. They have a reputation for absorbing oxygen more effectively than our koi, often leaving our fish picking up the crumbs of oxygen that our pond’s micro-organisms leave behind. This is particularly so in the warmer months when our koi are at their most active, feeding heavily. If oxygen were to become a limiting factor for our pond, then the water quality and the health of our koi is likely to deteriorate. The opposite is also true. Again from experience, if we add more oxygen, we can stimulate additional activity from our biofilter (that why trickle filters are more effective than wet filters) as well as from our koi. So even though aeration may be seen as an optional extra – if you want to get the most out of your pond, you should view aeration as a necessity.

Mixing. Depending on the aeration device you choose, you may benefit from a great deal of mixing as the air rises through the water column. This not only helps to reduce any dead spots inside your pond, but it will also help to maintain a consistent water quality through your pond, encouraging suspended solid matter to move towards your pump or bottom drain. In winter however, you should aerate with caution, as the associated mixing will lead to a chilling of the deeper layers.

Diffused aeration.


Easy and cheap to install

Aerates the complete water column

Mixes the pond water from the pond bottom upwards

Wide choice of different diffusers

One pump can drive many diffusers

Relatively cheap to run.


Might be difficult to find air pumps that will pump down to great depths

Diffusers can become blocked, requiring maintenance.

Can be a little unsightly when airline is visible inside a pond

Vigorous aeration can obscure view of koi.

Things to consider before buying:

There are many different types of diffusers available to the koi keeper, ranging from the basic (and disposable) airstone to other larger (and permanent) self-cleaning diffusers. Typical diffused aeration provides a constant stream of air bubbles, rising through the water column. A fine bubble size is essential to provide the maximum surface area for efficient oxygen transfer. For example, if a bubble size of 1/4″ is reduced to a fine bubble a quarter of that size (1/16″) the surface area is increased by a huge factor of 16! (The greater the surface area – the greater the rate of diffusion).

Even if some air diffusers look impressive by the sheer volume of large bubbles, they are relatively inefficient as they provide a relatively small surface area for gas exchange and most of the pump energy is used to blow air back into the atmosphere. Contact time between air and water as bubbles rise through the water column is also important. As water depth increases, contact time also increases combined with the increased head of pressure providing better physical conditions for dissolving oxygen into water.

Furthermore, if fine bubbles are generated by a suitable diffuser from depth, their large surface area creates a drag against the water, reducing vertical velocity increasing contact time further. An added benefit of this “drag” experienced by a rising column of fine bubbles is the mixing and circulation of significant volumes of water. This mixing action enhances water quality by making the pond a well-aerated, homogenous and stable environment. Fine bubble aeration also reduces surface agitation, permitting continued and undisturbed viewing of your koi.

Traditional air diffusers (airstones) are perhaps now regarded as inefficient because as the volume of air supplied is increased to increase aeration, the bubble size increases reducing gas exchange rates. They are also prone to clogging and blocking up, reducing airflow considerably. ‘Golf ball’ airstones are still a very handy and economical way for aerating biochambers and show vats.

A relatively new and unique flexible rubber diffuser (called an airdome) has been specifically developed to satisfy all the necessary criteria for optimum gas exchange. In addition the flexible nature of the unit has self-cleaning properties that ensure optimum efficiency during long-term use. This can be fitted onto a bottom drain where it aerates the pond from the bottom, giving enhanced aeration in addition to mixing of the whole water column, enhancing the removal of waste towards the bottom drain.



Cheap to run (if you can use existing pond pump)

No expensive hardware to purchase

No additional electrical installations


Might be difficult to install in an existing pond (usually plumbed through a wall)

Can restrict the flow of water from a pump

Minimum flow rates are required to create the required effect

Aeration looks vigorous (large bubble size), but in effect only aerates upper layers

Vigorous aeration can obscure view of koi.

Do require maintenance, and depending on the design, may prove difficult to clean.

This method is most easily installed through the wall of a pond from a final filter chamber. A venturi is a cylindrical hollow tube with a unique shape through which water is pumped. The unique shape is often referred to as an hour glass by which the diameter of the tube entrance tapers down to a smaller diameter, and finally returns to its original diameter.

The effect that the internal taper has, causes the velocity of the pumped water to increase. At the minimum diameter (or throat) of the venturi, the pressure drops only to eventually recover by the end of the venturi tube. If the pressure loss is sufficient at the throat, then a small hole at the throat connected to atmospheric air to be sucked into the returning pumped water, literally injecting air into the pumped returned water.

A venturi can look quite impressive as an aeration device, but in fact, compared to air pumped into the pond water directly, are relatively inefficient aeration devices. They are energy inefficient in that they involve the pumping of water (rather than air) and the bubble size is very large. This means that the rates of oxygen diffusion will be reduced. The large bubble size also means that only the upper layers of water receive the full benefit of the freshly oxygenated water.

Moving/ flowing water.


Cheap to run (if you can use existing pond pump)

No expensive hardware to purchase

No additional electrical installations

Water falls do not nee much maintenance


Might be difficult to install if you pond does not suit a waterfall

Only aerates upper layers

Vigorous water movement (especially fountains) can obscure view of koi.

Fountains can block relatively easily.

Just as sugar dissolves far better in a cup of coffee when it is stirred, so too oxygen dissolves better in water that is more highly energised (eg flowing, falling and cascading). In a natural environment, more energetic water courses are higher in DO than the slower ones, and besides the oxygen that is released by aquatic plants, water movement is the only method by which water can naturally become oxygenated. In a koi pond, a waterfall can be an effective means of aerating the water but in an intensively stocked pond, can lead to limitations. Oxygenated water that enters the pond at the waterfall will rarely sink to lower layers of water, being effective only at the surface. Furthermore, the physical work required to move water to oxygenate is significantly less efficient than moving air, especially when it only affects the upper layers of a pond.

Cost and Benefit – It’s all relative

Ultimately, we want the best aeration device for our koi pond in terms of its ability to aerate, but also with considerations for installation and cost. Fortunately, in the whole scheme of things, relative to the expense of a full koi pond set up, a powerful air pump and diffuser combination (my preferred method) are likely to be a minor cost – even when considering this is the most expensive of 3 aeration options.

Box out: Questions you should ask when looking to aerate using different methods.

a. Air pump

Depth to which the pump is required to deliver air?

The volume of air to be delivered at that required depth?

Is there a means of regulating the flow of air through to different diffusers at different depths?

Is there sufficient air to service the desired type of diffuser?

How many diffusers do you anticipate you will use?

Terms and length of guarantee

b. Diffuser type

What are your preferred diffusers (air stones, air domes etc)?

What will the alternatives cost?

Expected lifetime?

Maintenance required?

What is the diffuser’s performance when compared against others?

c. Venturi

Can it be retrofitted to my existing set-up?

How is it installed?

Will it fit with your existing or anticipated method of water return (It requires a pumped return)?

What is the minimum flow it requires to draw air through?

To what depth will it draw air through the venturi?

d. Moving water

Do you have space for a water fall?

Is a fountain suitable for your pond?

Can either a waterfall or fountain be incorporated with your existing pump and set up?

Combination for an average 2000 gallon pond:

For simplicity and overall effectiveness and robustness, I would opt for a simple air pump that is sufficiently powerful to pump air at the desired volume down to an air dome. Using this approach, you do not have any headaches of plumbing in a venturi (that is best suited to a pumped-return). Diffused aeration is also more efficient and turns over the whole pond volume. Cost of airdome £65, Air pump + airline £80.

Top factors and features that influence the effectiveness of an air pump:

1. Depth down to which it will pump

2. Volume delivered at that depth

3. Adjustable and robust manifold

4. Diffuser’s ability to deliver that air in fine bubbles

5. Diffuser’s durability. How soon will it block up / need replacing?

6. Terms and length of guarantee

7. The pump’s power consumption

Kill blanketweed and string algae.